1. Technical Field
The present application relates to a method and system for detecting a rotating wheel of a vehicle.
2. Background Art
Detecting vehicle wheels is of interest for numerous applications. For example, detecting wheels allows travel on a particular traffic area to be recognized with certainty, for example for border monitoring purposes or for triggering particular actions, such as triggering an alarm, activating lighting, opening a gate, recording a photo for monitoring purposes and the like. Modern traffic fee systems are also frequently directed to the number of axles of vehicles for fee assessment, so that the detection of wheels (wheel axles) can also constitute an important basis for imposing or checking road tolls, in particular also by way of mobile control vehicles, which are to check the number of axles of vehicles subject to tolls when passing them or in oncoming traffic.
It is known from DE 10 2008 037 233 A1 to detect wheels of a moving vehicle based on the different horizontal component of the tangential speed thereof as compared to the remaining vehicle, this tangential speed causing a corresponding Doppler frequency shift of a radar measuring beam. A radar speedometer is used for this purpose, which irradiates the lower region of passing vehicles by way of a radar lobe and time-averages a single speed measurement signal based on the received frequency spectrum that is returned, the signal exhibiting signal maxima at the locations of the wheels that are used for wheel detection.
The applicant of the present application disclosed novel and reliable methods for wheel detection that are particularly insusceptible to faults based on Doppler measurements in the not previously published patent applications EP 11 450 079.6, EP 11 450 080.4 and PCT/EP 2012/061645.
The applicant has recognized that the best possible alignment of the Doppler measuring beam with passing vehicles is desirable so as to further improve the detection reliability. On multi-lane roads, or roads with oncoming traffic, the distance at which a vehicle passes the detector unit frequently varies drastically, either due to driving style or the vehicle dimensions. This can result in insufficient illumination of the wheel to be detected by the measuring beam, causing detection errors.